Magnet system for an acoustic transducer

ABSTRACT

Magnet system for an acoustic transducer, in particular an electrodynamic microphone, which system comprises a main magnet and a ferromagnetic circuit the upper plate of which together with a centrally arranged core defines an annular effective airgap, the upper plate being provided with a centering member which preferably comprises at least 3 re-entrant projections and is integral with, and is made of the same material as, the upper plate. The re-entrant projections are moulded and calibrated in a single moulding operation during the manufacture of the upper plate. The re-entrant projections may be in the form of bridge pieces joined to a disk which is made of the same material.

United States Patent 11 1 Kleis June 24, 1975 I5 1 MAGNET SYSTEM FOR AN ACOUSTIC TRANSDUCER [75] Inventor: Derk Kleis, Emmasingel, Eindhoven, Netherlands [73] Assignee: U.S. Philips Corporation, New

York, NY.

[22] Filed: July 19, 1972 [21] Appl. No.: 273,299

[30] Foreign Application Priority Data 7/1966 Unitedf'Kingdom 179/180 163,965 2/1949 Austria l79/115.5 R 1,010,946 3/1952 France 179/117 336,913 4/l959 Switzerland 335/231 Primary Examiner-Kathleen H. Clafiy Assistant Examiner-George G. Stellar Attorney, Agent, or FirmFrank R. Trifari; Bernard Franzblau 7] ABSTRACT Magnet system for an acoustic transducer, in particular an electrodynamic microphone, which system comprises a main magnet and a ferromagnetic circuit the upper plate of which together with a centrally arranged core defines an annular effective air-gap, the upper plate being provided with a centering member which preferably comprises at least 3 re-entrant projections and is integral with, and is made of the same material as, the upper plate.

The re-entrant projections are moulded and calibrated in a single moulding operation during the manufacture of the upper plate.

The re-entrant projections may be in the form of bridge pieces joined to a disk which is made of the same material.

8 Claims, 5 Drawing Figures MAGNET SYSTEM FOR AN ACOUSTIC TRANSDUCER The invention relates to a magnet system for an acoustic transducer, in particular for an electrodynamic microphone, which system comprises a main magnet and a cooperating ferromagnetic circuit having a ferromagnetic upper plate which, together with a centrally arranged core, defines an annular effective airgap. The core is centered with respect to the upper plate by means of a centering member which is joined to the upper plate and the larger part of which is located at the lower end of the effective air-gap.

Such a magnetic system is described in Austrian Pat. No. 274,916. This magnet system, which is mounted in a microphone, comprises an annular main magnet provided with two pole pieces, a soft-iron upper plate and a soft-iron core rigidly secured to the lower plate. This core is centered by a separate centering ring made of a non-magnetic material which is glued to the lower surface of the upper plate.

The use of this additional ring, which must be mounted and glued, renders this magnetic system expensive and complicated.

It is an object of the invention to obviate this disadvantage, and a system according to the invention is characterized in that the centering member is integral with, and is made of the same material as, the upper plate.

The upper plate is provided with an inner edge formed with re-entrant projections during manufacture.

In a particularly advantageous embodiment the centering member comprises at least three re-entrant projections on the inner edge of the upper plate.

In this embodiment the upper plate is stamped from a sheet in a single operation, the re-entrant projections being calibrated either in the same operation or in a subsequent operation.

A further advantage in comparison with the known magnet system consists in that in the centering process only one tolerance is significant, whereas in the use of a separate centering ring, as is the case in the known magnet system, the tolerance of the mounting thereof is to be added to the said tolerance.

A magnet system according to a further embodiment of the invention is characterized in that each re-entrant projection takes the form of a bridge piece which is joined to the ferromagnetic part of the core opposite the upper plate and is integral therewith.

The magnet system comprises a magnet core on which a ferromagnetic disk is placed at the level of the upper plate. Both the upper plate and the disk are made in a single moulding operation, the disk and the upper plate being joined to one another by narrow bridge pieces.

To reduce the leakage of the magnetic flux via the projections or bridge pieces to a minimum, the crosssectional area of each of these elements is reduced so that the material is magnetically saturated at this point. As a result, the reluctance is considerably increased so that the magnetic leakage via the projections is greatly reduced.

Making the upper plate from sintered iron enables very close tolerances to be obtained during the moulding of the upper plate.

Embodiments of the invention will now be described, by way of example, with reference to the accompanying diagrammatic drawings, in which:

FIG. 1 is a cross-sectional view of an embodiment of a system according to the invention,

FIG. 2 is a plan view of the upper plate used in this system,

FIG. 3 is a cross-sectional view of the upper plate taken on the line [-1 in FIG. 2,

FIG. 4 is a cross-sectional view of another embodiment according to the invention, and

FIG. 5 is a sectional view taken on the line V--V of FIG. 4.

The magnet system shown in FIG. 1 is a system which is used in many types of electrodynamic microphones. It comprises an annular magnet I made of an oxidic ceramic substance, for example, ferroxdure. The magnet l is provided with an upper plate 2 and a lower plate 3 which is integral with a core 4. The inner edge 5 of the upper plate 2 together with the upper edge 6 of the core 4 defines an effective air-gap 7. This air-gap serves to receive a moving coil, not shown, which is secured to a diaphragm, not shown. The lower plate 3 and the core 4 are made of soft iron.

The upper plate 2 (see FIGS. 2 and 3), which is made of sintered iron, is provided with 3 re-entrant projections 8, the end of each projection having a reduced cross-sectional area. The ends 8 are bent downward through a distance such as to allow sufficient space for the largest deflection which the moving coil may perform in operation.

The inner edges 10 of the ends 9 are calibrated. During the assembly of the magnet system the upper plate 2 is centered with respect to the core 4 by the three projections 8.

FIGS. 4 and 5 show another magnet system which comprises a centrally arranged permanent magnet 11 made of Ticonal. This magnet is provided with an upper plate 12 which through three re-entrant projections 18 is joined to a disk 14 which forms a pole piece for the magnet 11 on the upper surface thereof. The projections 18 are in the form of bridge pieces which have parts of reduced cross-sectional area.

[n the two magnet systems according to the invention, the ends 9 and the bridge pieces 19, respectively, have portions the cross-sectional areas of which are reduced so that the centered material is saturated at these points, with a consequent very high reluctance.

The upper plate 2, and the upper plate 12 together with the disk 14, respectively, are moulded in one operation in which the inner edge 10 and 15, 16 respectively, are calibrated to have the required dimensions.

What is claimed is:

l. A magnet system for an acoustic transducer comprising, a main magnet and a ferromagnetic circuit which cooperates with the main magnet, said ferromagnetic circuit including a centrally arranged ferromagnetic core and a ferromagnetic upper plate having an opening therein which; together with the centrally arranged core, defines an annular effective air gap, said upper plate including a centering member integral therewith and made of the same ferromagnetic mate rial as the upper plate and comprising at least three circumferentially arranged reentrant projections on the inner edge of the upper plate with a part of each projection being dimensioned so that in operation said projection part is magnetically saturated, said core being centered wtih respect to the upper plate by means of the centering member the larger part of which is located at the lower end of the effective air gap.

2. A magnet system as claimed in claim 1 wherein the cross-sectional area of said projection part is dimensioned to produce said magnetic saturation thereof.

3. A magnet system as claimed in claim 1, characterized in that the material of the upper plate comprises sintered iron.

4. A magnet system for an acoustic transducer comprising, a main magnet and a ferromagnetic circuit which cooperates with the main magnet, said ferromagnetic circuit including a centrally arranged ferromagnetic core and a ferromagnetic upper plate having an inner portion approximately coaxial with the core and adjacent to one end of said centrally arranged core and a peripheral portion with an opening that defines an annular effective air gap separating the inner and peripheral portions of the upper plate, said upper plate including a centering member for centering the core with respect to the upper plate and comprising at least three circumferentially spaced bridge members integral with the inner and peripheral portions so that the parts of the upper plate form a unitary piece and with all parts thereof made of the same ferromagnetic material.

5. A magnet system as claimed in claim 4 wherein a part of each bridge member is dimensioned so that in operation said part is magnetically saturated.

6. A magnet system as claimed in claim 5 wherein the larger part of the bridge member is located at the lower end of the effective air gap and its cross-sectional area is dimensioned at said part to produce the magnetic saturation thereof.

7. A magnet system as claimed in claim 4 wherein the magnet system comprises a permanent magnet core and the inner portion of the upper plate is shaped like a disk and is aligned with the core and all parts of the upper plate are formed as a single integral piece of the same material.

8. A magnet system as claimed in claim 4 wherein said bridge members extend in a generally radial direction bridging the lower parts of said air gap. 

1. A magnet system for an acoustic transducer comprising, a main magnet and a ferromagnetic circuit which cooperates with the main magnet, said ferromagnetic circuit including a centrally arranged ferromagnetic core and a ferromagnetic upper plate having an opening therein which; together with the centrally arranged core, defines an annular effective air gap, said upper plate including a centering member integral therewith and made of the same ferromagnetic material as the upper plate and comprising at least three circumferentially arranged reentrant projections on the inner edge of the upper plate with a part of each projection being dimensioned so that in operation said projection part is magnetically saturated, said core being centered wtih respect to the upper plate by means of the centering member the larger part of which is located at the lower end of the effective air gap.
 2. A magnet system as claimed in claim 1 wherein the cross-sectional area of said projection part is dimensioned to produce said magnetic saturation thereof.
 3. A magnet system as claimed in claim 1, characterized in that the material of the upper plate comprises sintered iron.
 4. A magnet system for an acoustic transducer comprising, a main magnet and a ferromagnetic circuit which cooperates with the main magnet, said ferromagnetic circuit including a centrally arranged ferromagnetic core and a ferromagnetic upper plate having an inner portion approximately coaxial with the core and adjacent to one end of said centrally arranged core and a peripheral portion with an opening that defines an annular effective air gap separatingg the inner and peripheral portions of the upper plate, said upper plate including a centering member for centering the core with respect to the upper plate and comprising at least three circumferentially spaced bridge members integral with the inner and peripheral portions so that the parts of the upper plate form a unitary piece and with all parts thereof made of the same ferromagnetic material.
 5. A magnet system as claimed in claim 4 wherein a part of each bridge member is dimensioned so that in operation said part is magnetically saturated.
 6. A magnet system as claimed in claim 5 wherein the larger part of the bridge member is located at the lower end of the effective air gap and its cross-sectional area is dimensioned at said part to produce the magnetic saturation thereof.
 7. A magnet system as claimed in claim 4 wherein the magnet system comprises a permanent magnet core and the inner portion of the upper plate is shaped like a disk and is aligned with the core and all parts of the upper plate are formed as a single integral piece of the same material.
 8. A magnet system as claimed in claim 4 wherein said bridge members extend in a generally radial direction bridging the lower parts of said air gap. 